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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />reduction curve), and the <br />applied to each subbasin. <br />shown in Table 1. <br /> <br />6-hour rainfall distributions were developed and <br />The 6-hour rainfall depths for each subbasin are <br /> <br />Three methods were used to estimate rainfall losses; 1) Green and Ampt <br />infiltration equation with a surface retention loss, 2) Initial Loss plus <br />Uniform Loss Rate (IL+ULR), and 3) the CN method. The Green and Ampt infil- <br />tration parameters were estimated by procedures contained in the Maricopa <br />County, Arizona, Hydrologic Design Manual. A description of this rainfall <br />loss procedure and methods to estimate the parameters is contained in Appendix <br />A. The Green and Ampt infiltration equation is a three parameter model and is <br />a decay type function that is based on accumulated soil moisture and the <br />antecedent soil moisture condition. The parameters can be estimated according <br />to soil texture classification. Based on field observation and experience, <br />the soil in the watershed was classed as loam. <br /> <br />The IL+ULR parameters were estimated by the methods in the Maricopa <br />County Hydrologic Design Manual. Reports of previous flood hydrology studies <br />in the watershed were researched and it was found that a uniform loss rate of <br />1.0 inch per hour and an initial loss of 0.05 inch has been used (Muller <br />Engineering Company, Inc., 1983). An uniform loss rate of 1.0 inch per hour <br />is judged to be too high and an initial loss of 0.05 inch is judged to be too <br />low. As a comparison, Mr. Fred BertIe, retired head of the USBR Flood Hydrol- <br />ogy Section, recently performed a flood study of Dillon Dam for the Denver <br />Water Department (BertIe, 1982). In that study, Mr. BertIe reconstituted <br />several historic storms in the Blue River basin. Those reconstitutions <br />resulted in uniform loss rates of about 0.25 to 0.40 inch per hour. It is <br />likely that the Blue_River watershed would have loss rates comparable to the <br />Boulder watershed. <br /> <br />The SCS Curve Number (CN) method (SCS, 1972) was used for comparison. A <br />CN of 78 was selected as the "best" estimate and a CN of 68 was also used to <br />demonstrate the sensitivity of the CN method. <br /> <br />2 <br />